In general, drugs which are slightly soluble in water and have high crystallinity have low bioavailability since they have low solubility and low disolution rate in the gastrointestinal tract. Hitherto, in order to improve their absorbability, several methods for finely grinding drug crystals or for transforming them into amorphous substances have been examined.
By fine-grinding, however, particle diameters become irregular between lots, or inter-particle force is enhanced to cause agglomeration.
As a method for obtaining amorphous substances, grinding or forming a solid dispersion are considered. Drug crystals which can become amorphous by grinding are limited. Further, it has been known that solubility and absorbability of a slightly water-soluble compound are improved by dispersing it in a polymer to form a solid dispersion.
As to xanthine derivatives or pharmacologically allowable salts thereof, which are slightly soluble, however, solid dispersions or solid dispersion dosage forms are not yet known.